1. Field of the Invention
The present invention relates to a method for manufacturing of supplementary cementitious materials (SCMs), i.e pozzolans.
2. Description of the Related Art
Fly ash is a supplementary cementitious material, which is useful for production of concretes, mortars and other mixtures comprising cement. Fly ash is a by-product of coal burning power plants and is produced worldwide in large quantities each year.
In the Swedish patent No 532790 a method is described which eliminates fluctuations in the fly ash quality due to variations in the coal chemical compositions and parameters of coal burning processes. Processing of pozzolans, i.e. fly ashes according to the patented method significantly improves performance of the concrete and provides higher level of replacement of Standard Portland cement, which leads to significant economical and environmental benefits.
Fly ash usually contains about 85% glassy, amorphous components in the form of cenosphere particles. According to ASTM C 618 fly ash is classified in two classes, Class C and Class F. The Class F fly ash typically contains more than 70% by weight of silica, alumina, and ferric oxides, while Class C typically contains between 70% and 50%. Class F is produced as a by product of the combustion of bituminous coal. Class C fly ash has a higher calcium content and is produced as a by-product of the combustion of sub-bituminous coal.
Comprehensive research has demonstrated that a high volume fly ash concretes, where Portland cement has been replaced by fly ash to a level over 50% showed a higher long term strength development, a lower water and a gas permeability, a high chloride ion resistance, etc. in comparison with Portland cement concretes without fly ash.
At the same time a high volume fly ash concrete has significant drawbacks. One drawback is long setting times and unsatisfactory slow strength development during the period 0 to 28 days. These negative effects significantly reduce the level of fly ash used for replacement of Portland cement, e.g. about 15% in the US.
Serious problems are also related to the stability of fly ash performance. Usually variations in the chemical composition of coal and frequent changes in operating parameters of power plants cause, among others, formation of crystalline and quasi-crystalline phases, so called scoria, which leads to reduction in fly ash reactivity, so called pozzolanic activity.
Historically limestone-pozzolan mixes have been used for 2000 years ago by ancient Romans and a lot of ancient buildings as e.g. Coliseum are still in a good shape. At the same time despite the fact that according to the US Geological Survey billions of tons of natural pozzolan can be found in Western USA and in most other regions of the world. The usage of natural pozzolans in concrete in the modern building industry is very limited.
Natural pozzolans belong to materials of volcanic origin and sedimentary origin, such as diatomaceous earth. According to ASTM C 618 they are designated Class N pozzolans. The natural pozzolans are described in ACI 232.1R-00 as “Raw or calcinated natural pozzolans that comply with the applicable requirements for the class herein, such as some diatomaceous earth; opaline chert and shales; tuffs and volcanic ashes or pumicites, any of which may or may not be processed calcinations; and various materials requiring calcination to induce satisfactory properties, such as some clays or shales.”
The reason why the use of natural pozzolans is very limited can be explained by the following.
The microstructure of natural pozzolan particles is characterized by high porosity, which significantly increases water demand of concrete mixes containing natural pozzolans to achieve required flowability/workability of the concrete. However, increases in water demand lead to unsatisfactory compressive strength development.
Additionally uneven distribution of active minerals i.e. amorphous materials, e.g. due to weathering has a further and unacceptable negative impact on negative strength development of concrete with natural pozzolan content.
Optimized natural pozzolan particle size distribution is required in order to achieve consistent performance of high pozzolan containing concrete. An optimized particle distribution is such that there are fine and coarse fractions present.
According to the Report of American Concrete Institute ACI 232.1R-00 “Use of raw or processed natural pozzolans in concrete”, properties of natural pozzolans can vary considerably, depending on their origin and therefore variable proportions of chemically active minerals. Chemically active minerals usually contain amorphous materials (e.g. amorphous silicon dioxide), which react with calcium hydroxide released during Portland cement hydration (pozzolanic reaction) and form calcium-silicate-hydrate gel [C—H—S gel]—a product similar to the resulting product of Portland cement hydration.